This invention pertains to the confinement of plasmas by magnetic fields. More particularly, the invention is directed to a major advance for open-ended magnetic confinement devices.
Apparatus employed for the containment of plasmas by magnetic fields may utilize many varied configurations. Two well-known categories of these machines are the open-ended type, such as the magnetic mirror, and the toroidal type, such as the tokamak and the stellarator. One advantage to the toroidal type is that a trapped charged particle must move laterally across magnetic field lines to escape confinement. Hereinafter, positive ions will be designated simply as "ions." Since the ions tend to remain in a spiral orbit about a given set of magnetic field lines, the continuity of the magnetic field lines inside the apparatus enhances containment.
An apparatus of the open-ended type has the disadvantage that the trapped charged particles may escape while traveling along the magnetic field lines which define their spiral orbits. The magnetic field lines do not close upon themselves inside the simple magnetic mirror. Some, but not all, ions are reflected by an increased magnetic field strength at the mirror throat. As a result, the simple magnetic mirror suffers large plasma losses through the mirror ends. The net positive potential of the confined plasma adds to the losses since ions are confined better than electrons in a simple magnetic mirror. One early mirror confinement apparatus is disclosed in Post, U.S. Pat. No. 3,170,841, filed on July 14, 1954. Post is hereby incorporated by reference. The physics of a simple magnetic mirror is discussed at length in the Post patent as well as in Samuel Glasstone and Ralph H. Loveberg, Chapter 9, "Magnetic Mirror Systems," Controlled Thermonuclear Reactions, D. Van Nostrand Co., Inc., Princeton, N.J. (1960), p. 336 et seq and in David J. Rose and Melville Clark, Chaper 10, "Motion of Individual Charges," Plasmas and Controlled Fusion, John Wiley & Son, Inc., New York (1961) p. 198 et seq.
The problem of end losses in magnetic mirrors has been addressed in a number of ways. One approach links several mirrors together to form roughly a toroidal configuration with magnetic field lines closed inside the apparatus. Particles which leak out of one magnetic mirror simply leak into an adjacent magnetic mirror. Post noted this in FIG. 25 of U.S. Pat. No. 3,170,841, supra. Other closed systems of linked magnetic mirrors include Dandl, U.S. Pat. No. 3,728,217. Each magnetic mirror segment is independent of the next, the total effect on the toroidally confined plasma being a stabilization due to the bumpy nature of the toroidal magnetic field together with disks of hot electrons spaced in mirror cells around the toroidal plasma. These electrons are heated by microwave radiation at the election cyclotron resonance frequency. In either patent there is no teaching of electrostatic plugging of adjacent mirror cells. The same may be said for other linked mirror systems such as the Christofilos, U.S. Pat. No. 3,668,067. In both Dandl and Christofilos, the linked magnetic mirrors are secondary to the stabilization and confinement of the plasma, which is by means other than by electrostatic plugs.
In linked three-cell systems, the earliest prior art appears in FIG. 22 of Post, U.S. Pat. No. 3,170,841. However, Post's three-cell system does not operate as three cells simultaneously. The end cells exist as thermonuclear reaction zones alternately and do not serve to electrostatically stopper the central cell.
A three-mirror system to change the potential at the linking magnetic mirrors is suggested by G. G. Kelley, Plasma Physics 9, 503 (1967). Since electrons travel more freely through the mirroring regions than ions, the mirroring regions have a net negative charge. Thus, ions which would have mirrored are drawn deeper into the mirroring region, and some are lost. To overcome this enhanced end loss, Kelley injected cold neutral species into the mirroring regions of the center mirror cell of a three mirror cell system. The cold neutral species ionize; thus, these mirroring regions substantially lose their negative potential. Kelley did not try to make the end mirror cells electrostatic end plugs to stop end losses in the center mirror cell. He addressed a problem of enhanced end losses without touching on the basic end-loss problem in an open-ended system.
The use of magnetic mirror cell plasmas to end stopper ions in a center mirror cell is disclosed in U.S. Pat. No. 4,125,431 issued Nov. 14, 1978, to T. Kenneth Fowler. Also different modes of start up procedures for mirror cells and designs of current-carrying coils for mirror cells are described for the Fowler three mirror cell system. The electrons are free to travel in each of the three mirror cells, thus effectively giving rise to one electron temperature. Electrons from the hotter plasma of the end mirror cells heat the center mirror cell, which thus increases the number of ions with the energy for escape from the potential well formed by the end mirror cells. One method of easing this problem by using charge exchange cooling is disclosed in U.S. Pat. No. 4,127,442 which issued Nov. 28, 1978, to B. Grant Logan. However, the density and energy of the end mirror cell plasmas must be kept higher than in the center mirror cell to achieve the end stoppering. As the energy of ions to be confined in the center mirror cell goes up, the required density and energy for end stoppering by the end mirror cells increase even more. Thus, for a reference design tandem mirror with 30 keV center mirror cell plasma, the end mirror cell mean ion energy is 880 keV maintained by 1.2 MeV neutral beam injectors in a 16.5 tesla mid-plane magnetic field. (See Lawrence Livermore Laboratory Report UCRL-79092 Summary, available at the National Technical Information Service). Such high neutral beam injector energies and intense magnetic fields create a problem in capital cost and energy used to maintain the plasma confinement.
The abstract "Suppression of the Ion Cyclotron Instability in Phoenix II Using Electron Transit Time Heating" by E. Thompson, et al, in the American Physical Society Tenth Annual Meeting of the Division of Plasma Physics on Nov. 13-16, 1968, at Miami discloses heating of mirror cell electrons by rf fields at the electron transit time frequency. Thompson et al was able to suppress some instabilities. They did not note a change in mirror cell potential as a result of this heating.
Thus, the ion end losses have been solved for open ended systems by Fowler's use of mirror cell plasmas to end stopper each end of a center mirror cell. No other part of the art taught end stoppering by use of plasmas. Yet, the small intense end mirror cells of Fowler's tendem mirror are classical mirror cells with end losses of ions and electrons. For electrostatic end stoppering, the demands on the end mirror cells rise greatly with the energy of the ions to be confined in the center mirror cell. An excellent example of where these demands rise greatly with the energy of the ions is shown above for 1.2 MeV neutral beam injectors at the sixteen and one-half tesla end mirror cells confining a center mirror cell plasma of 30 keV. The tandem mirror improved the position of mirror cell systems with respect to toroidal plasma confinement, but the energy expended and capital cost incurred for a 30 keV center mirror cell plasma still leaves great room for improvement in open-ended magnetic confinement and many of these problems had not been improved upon until the applicants' invention described herein.